Many systems for forced air ventilation using thermostatically controlled fans are known in the prior art. Generally, the fans are driven by electric motors which are either switched fully on or fully off. The losses associated with this switching action increases the energy required to operate these systems. Additionally, mechanical shutters or air bypasses are used to limit or to divert the air flow while the fans operate at maximum speed. Such shutter systems are uneconomical because energy is wasted moving air which is not used directly for ventilation purposes.
For example, U.S. Pat. Ser. No. 2,963,225 (Shelton, 1960) shows the use of thermostats and timers to control motor driven louvers which are used to restrict air flow through ventilating registers.
Alternatively, potentiometers are used to vary the voltage applied to electric motors, as in U.S. Pat. Ser. No. 2,351,695 (Newton, 1944). Adjustment in voltage produced by the potentiometer acts to vary the speed of the electric motors, but the potentiometer also causes uneconomical resistive losses. Solid-state devices such as silicon-controlled rectifiers are also used in the prior art to control the speed of electric motors by limiting the applied voltage. The rectifiers eliminate the resistive losses of the potentiometer, but deform the sinusoidal wave form of the alternating current used to drive the motors. Electrical losses thus occur because the wave form no longer coincides with that on which the electric motor was designed to operate.
The system of the present invention eliminates these disadvantages by allowing the speed of the fans to be varied through a range of air-moving capacities. Unnecessary cycling or diversion of air is completely eliminated and the accompanying temperature over- and under-swings are greatly reduced. In addition, the single pump drive motor achieves greater efficiencies by minimizing losses associated with use of multiple drive motors.